In the navigation of an aircraft along a route to a specified destination, various types of equipment are available to the pilot to aid in the navigation of the aircraft to that destination, including rather-sophisticated navigational computer systems which are capable of providing such information as heading, altitude, geographical position, ground speed, and the like. However, pilots still rely to a large extent on the visual observation of check points along the route to the selected destination. Thus, the visual observation of cultural terrain features such as rivers, lakes and natural rock formations, as well as man-made cultural features such as roads, towns and significant structures, provide an important guide to the pilot in his navigation of the aircraft.
On the other hand, as a result of weather conditions, it may not be possible to visually observe the terrain for these cultural checkpoints, the visibility of which is also impaired to a great extent during hours of darkness. Thus, it is highly desirable to make available to the pilot some means of picking out cultural checkpoints along the path of the aircraft as it proceeds towards its selected destination. To this end, various systems have been proposed, including radar scanning systems and systems using preprocessed films of terrain over which an aircraft is to pass for providing to the pilot a display which simulates that which he would visualize if he were to actually view the terrain over which the aircraft is passing.
One of the most-recent developments in the area of terrain display systems relates to a system for the real-time dynamic display of terrain data which is stored in compressed digital form and which may be viewed on a cathode ray tube display in the form of a moving map that is automatically oriented under the control of the aircraft's navigational computer system to the instantaneous position of the aircraft with a heading-up disposition. This system is disclosed in copending U.S. application Ser. No. 224,742, filed Jan. 13, 1981, now abandoned, which application is entitled "Digital Map Generator and Display System", filed in the name of Paul B. Beckwith, Jr., and is assigned to the same assignee as the present application.
The system disclosed in the above-mentioned copending application provides a topographical two-dimensional display of the terrain over which the aircraft is passing on the basis of digital data stored on a cassette tape as data compressed in the transform domain, thereby permitting the storing of large quantities of terrain data for use in generating the moving map display.
The compressed terrain data is read from the cassette tape as the aircraft traverses the terrain to provide instantaneous access to that data relating to the terrain over which the aircraft is passing and this data is reconstructed by decompression processing and is stored in a scene memory with a north-up orientation. The terrain data is then read out of the scene memory in a direction related to the heading of the aircraft, the elevation data thus read out is processed to provide slope shading and a video display in the form of a real-time moving map of terrain is generated therefrom.
In the above-mentioned display system, the addition of cultural data to the real-time display even more particularly enhances the realistic aspects of the information available to the pilot and provides the check points in the display which aid the pilot in the navigation and guidance of the aircraft. In the system described in U.S. application Ser. No. 224,742, it is proposed that the elevational data be compressed in accordance with a hybrid discrete cosine transform (DCT) compression algorithm, with differential pulse code modulation (DPCM) being used to transmit the DCT coefficient differences between each row of grid points. In reconstruction of this data, all of the in-between picture element (pixel) points are filled in by interpolation directly from the DCT transform in one direction and in the other direction, the DPCM data is interpolated to provide data to fill in intermediate lines. However, for cultural data which occupies only selected portions of the display area, this type of compression entails undesirably-large storage requirements for the relatively-small amount of data involved. For example, for a 512.times.512 pixel display of cultural having a single river or road as terrain data extending across a portion of the display, it would not be efficient to store all 262,144 pixels even in a compressed form simply to display what would appear as a single line in the display.
One solution to the problem of compression and storage of cultural data in a moving map display is based upon the display of these cultural features as individually-generated characters with different types of cultural features being designated by different digital values. In this regard, a footprint technique of generating lines in a video display may be used in which the data for the display is stored in the form of an address of the beginning point of the line to be generated and subsequent delta code values defining the incremental steps in respective X and Y directions for generating line segments from the beginning point through successive points on the line to the end point thereof. In accordance with the footprint scheme of line generation, as each point on the line is generated, the next point is determined on the basis of a delta code (.DELTA.X,.DELTA.Y) which is capable of identifying the footprint or pattern of points in an area surrounding the last point generated. Thus, where the delta code consists of eight bits, the footprint will consist of a pattern of 16.times.16 points symmetrically disposed with respect to the last point generated along the line. However, with this type of scheme, the maximum spacing of points along the generated line on the basis of the eight bit delta code will be approximately eight points, which is the distance between the center and one corner of the square pattern surrounding the last point generated. Thus, the degree of compression which can be achieved using this footprint technique is limited.